Angioplasty procedures are well known within the medical community. During such a procedure, a catheter is navigated through a lumen of the human body to a site needing expansion. For example, a distal portion of a catheter containing a deflated balloon is directed to an area of an artery that is substantially blocked, and that can be enlarged upon expansion of the balloon.
Balloon catheters include pneumatically and hydraulically actuated catheters. Traditionally, catheters have employed hydraulic means of expansion. A balloon catheter may be manufactured from an elastomeric conduit with an enlarged diameter portion thereof forming a balloon. Upon the balloon reaching the procedure site, pressurized fluid is directed through the conduit and to the balloon so as to enlarge the diameter of the balloon, thereby imparting force against the interior walls of the lumen and thus expanding the blocked area. In order to minimize the entrance diameter of the puncture hole through the skin into the arterial system and thereby decrease the time for healing, as well as the amount of scar tissue after healing, it is desirable to be able to reduce the diameter of the balloon catheter system while non-pressurized.
Currently such angioplasty catheters are made using either compliant or non-compliant elastomeric material. Due to the necessity of being able to use high forces to open up blocked arteries, fluid pressures used to actuate the balloon can be very high (more than 20 atmosphere). With a compliant balloon material, this requires very thick elastomeric materials to be used. Thick compliant materials can withstand such high pressures, and adequately retract into their original dimension to allow for retraction through the lumen, however, their thickness is counter to the desire of having non-expanded small dimensions. Non-compliant balloon materials can be constructed having thinner balloon wall dimensions, but use of such materials generally entails balloon folding for size reduction when not expanded.
Moreover, both compliant as well as non-compliant balloon materials generally have to be hydraulically actuated, therefore one has to provide a fluid access lumen through the complete catheter system. The walls of such an access lumen have to be sufficiently strong to withstand the pressure, but this design demand is in contrast to the use of highly flexible, thin catheter systems that allow for optimization of push and track without bursting.
A further downside of hydraulic balloon actuation is the risk of balloon leakage. Leaks can originate during expansion in calcified lesions. As the creation of leaks will prevent further expansion, this can lead to very serious situations, for example, with a balloon expanded stent procedure, this can lead to a partially-deployed, and thus unstable, stent. As a result, the catheter often has a thick and bulky shaft-like construction. For most applications, small diameters and high flexibility are of great importance. For example, with neurological procedures, or procedures within the lower extremities having mostly torturous vessels, such fluid-driven elastomeric catheters and their relatively large diameters, are simply unusable. Further complicating matters is the fact that such balloon constructions can only be made to a certain minimum diameter, thus preventing usage in such lumens, as well as lumens which have been reduced to a small diameter due to a condition requiring the angioplasty. Especially challenging is the use of current balloon catheters when stenting a bifurcation. Bifurcation generally involves using two balloons in parallel, doubling the space requirements.
Medical device needs are not limited to balloon catheters. The rapid increase of use and importance of implantable medical devices, e.g., stents, in cardiology and other medical fields calls for new and improved methods and devices for delivery and retrieval of such implants. New and improved means of drug delivery allowing more precise and controlled release of pharmaceuticals are also sought.
With the miniaturization and increase in complexity and functionality of medical devices, there exists a need for improved micro-actuation technologies. Existing electro-active materials that are appropriate for use in medical device actuators include electro-active polymers (EAPs), electroactive ceramics (EACs) and shape memory alloys (SMAs). However, each of these technologies has significant limitations. While having the ability to induce strains that are as high as two orders of magnitude greater than the movements generally possible with the relatively rigid and fragile EACs, EAPs have relatively low actuator forces and mechanical energy density; some EAPs also show a lack of robustness. SMAs are fairly rigid, only have strains that reach 8%, and they do not have full reversal action.
Collectively, the current failings of balloon catheters and other medical devices, as well as the limitations of actuators appropriate for use in such devices, reveals the need for novel medical devices incorporating new actuator technologies.